The present invention relates to a light exposure mask for use in a lithography step of a semiconductor manufacturing process and, in particular, a light exposure mask having a phase shift pattern.
Further, the present invention relates to a substrate for forming an exposure mask and a method for forming this light exposure mask.
With a recent advance in the semiconductor technique a semiconductor device or element has been so advanced as to offer a high-speed device of a high integration density. These recent advances have resulted in an ever-increasing need for very fine patterning of semiconductor devices and elements, and for patterns having very small configurations and high integration densities.
In order to satisfy such a need, the light of a short wavelength such as far-ultraviolet light has been used as an exposure light source. In recent years, attempts have been made to achieve such a microminiaturization without changing an exposure light source. As one technique there is known a phase shifting method. This phase shifting method comprises forming, at a light transmitting section, partially a phase inversion layer for allowing an optical path length to be delayed by .lambda./2 and, by doing so, eliminating any adverse effect caused by the diffraction of light produced between adjacent patterns and hence achieving an enhanced pattern accuracy.
Of the phase shifting method there is known an alternative phase shifting method for improving an image resolution in particular. In this technique, as shown in FIG. 1A, a light shielding film 102 with openings (light transmitting sections) provided therein is formed on a light transmitting substrate 101 in a manner to provide a phase shifter 103 in one of the adjacent openings and not in the other. The phase of the light passing through the phase shifter 103 is 180.degree. inverted relative to the light passing through the section where the phase shifter 103 is not provided. Through the phase inversion of the light rays in the adjacent openings, a negative interference occurs between the adjacent light rays, thus improving a resolution performance.
The alternative phase shift mask can also be provided in a manner to form one (for example, 105) of adjacent openings 104, 105 as a groove in a substrate 101 relative to the other as shown in FIG. 1B. In this structure, even if these openings 104, 105 have the same dimension, there arises the problem of producing a difference in light intensity between the opening (groove) in the substrate and the other opening on the substrate, that is, between a phase shift section 105 and a non-phase shift section 104. Namely, through an interference between the edges of a pattern located substantially parallel to an optical axis an opening size is optically narrowed at the phase shift section 105.
In order to solve this problem, a proposal has been made to provide both adjacent openings 106 and 107 as grooves in a substrate as shown in FIG. 1C, so that it is possible to achieve a uniform light intensity through an interference at the edges of a pattern between the openings.
In FIG. 1C, a chromium compound is principally used as a light shielding film 102. The composition of this compound is adjusted, when in use, so as to reveal the light shielding property not only in an exposure light wavelength range but also in a range below a visible area. In order to allow light rays which transmit one (first groove) of the openings 106, 107 and the other (second groove) to be set in a phase inversion relation, amounts of cut in the openings (grooves) are adjusted. The difference in amounts of cut in the first and second grooves 106 and 107 is set as an optical path length difference so that it is made equal to substantially one half the exposure light wavelength .lambda. (phase shift thickness). Further, the amount of cut (bias amount) in the shallower groove is so set as to be substantially equal to the difference in cut amounts between the grooves 106 and 107.
However, this type of exposure light mask involves the following problem. That is, the existing light shielding film, being light-shielding in property, has its light intensity transmittance not made zero and has a transmittance of 0.01 to 0.1%. This phenomenon will occur so long as the extinction coefficient of the light shielding film is not infinite.
FIG. 2 shows a focal point tolerance found from an optical image of an alternative pattern in the case where, as the light shielding film, use is made of a light shielding film of 0.1% transmittance whose optical path length of light passing therethrough is lengthened by .lambda./4 (has a phase difference of 90.degree.) compared with an adjacent medium having the same thickness as that of the light shielding film. By the focal point tolerance is meant an ordinary term defined by a range in which the forward/backward shifting of the focal point is allowable under a precision limit (the tolerance of the pattern dimension).
The target pattern dimension was 0.15 .mu.m and the phase difference of the adjacent openings at the substrate was 180.degree.. In FIG. 2, the abscissa represents an exposure light amount (dose) in logarithm and the ordinate a defocus amount. The precision limit was set to be .+-.10% with respect to a desired value.
Here, a curve 201 represents the exposure light amount against the defocus amount in the case where the dimension varies in a range of .+-.10% against light passing through a substrate surface. Similarly, a curve 202 represents a similar exposure light amount (dose) of a 180.degree. phase pattern adjacent to a pattern with a substrate surface defined as an opening. An area defined by these curves 201 and 202 shows an area falling within .+-.10% of a desired dimension.
In the alternative phase shift mask, if the transmittance of the light shielding film is set to be zero, those curves corresponding to the two openings are completely superimposed with respect to each other as shown in FIG. 3 (see the curve 301). In FIG. 2, the failure of the curves 201 and 202 to be superimposed with respect to each other is because the light shielding film has a transmittance and a phase involved.
Here, consideration is paid to a variation in sensitivity of a resist, illumination spots by the light exposure device, and so on, and their variation values are set to be 10% (the dose variation conversion values). An area satisfied even if the line width variation amount as set out above produces this dose variation conversion value is represented by a black frame 203 in FIG. 2 and a black frame 203 in FIG. 3. In the case where the phase of the light shielding film was 90.degree. as in FIG. 2, there occurred a 30% fall in depth of focus in comparison of 0% in light intensity transmittance of the light shielding film.
The following is a table on data at respective wavelengths of the heretofore used light shielding film material (CrOx/Cr), that is, data on a refractive index, an extinction coefficient and a film thickness and a consequent transmittance, phase relation and deterioration in depth of focus in the case where this film is applied to an alternative phase shift mask.
TABLE __________________________________________________________________________ Cr film CrOx film deterio- film- film ration refrac- extinc- thick- refrac- extinc- thick- trans- in focal wave- tive tion tive tion mittance phase point length index factor (nm) index factor (nm) (%) tolerance __________________________________________________________________________ 248 nm 1.62 1.75 700 2.04 0.94 300 0.051 98.69 23% 365 nm 1.85 2.38 700 2.34 0.81 0.151 86.00 43% 193 nm 1.85 2.75 700 2.73 0.71 0.242 79.82 52% __________________________________________________________________________
From this Table it is found that, for any wavelength, the phase difference is not adjusted and has a value near to 90.degree.. In the case where, for these wavelengths, respective light shielding films were applied to the alternative phase shift mask, a prominent degeneration occurred in terms of the depth of focus.
In this way, although, in the conventional alternative phase shift mask, the transmittance of the light shielding film is dealt with as 0, it has about a 0.1% (OD=3) in actuality and it has, therefore, not been possible to achieve a performance inherent to the alternative phase shift method. When, in particular, the phase difference between the light passing through the light shielding film and the light passing through a light transmitting medium of a corresponding thickness was 90.degree., the focal point was found to be largely narrowed.